A portable communication terminal, e.g. a mobile phone, typically comprises at least one speaker, or transducer, adapted to be held to a user's ear for providing sound during speech communication with a remote party. Such a speaker, herein called an earphone speaker, is therefore configured to cooperate with the acoustic compliance provided by the cavity formed by the outer ear when held over an output for the speaker. A general trend among portable devices in general, and mobile phones in particular, is miniaturization. The technologies surrounding manufacture of batteries, antennas, and electronics have developed rapidly over the last decades, making it possible to provide pocket size mobile phones with ever increasing performance. However, at some point there will be contradictory interests, in that the terminal should be as small as possible, and at the same time have an attractive user interface. The progress in display technology has therefore had a large impact on mobile phone design, and today all major mobile phone manufacturers provide terminals with high performance multi-color displays. So, as the displays get larger and an overall compact terminal is desired, the layout of the terminal has to be looked over.
The audio interface of the mobile phone includes a microphone and a speaker. However, these elements provide no visual advantage, and should therefore be a small as possible. For the speaker, this is a problem since the smaller the diameter or area of the speaker the higher the acoustic compliance, and a speaker with a high compliance membrane will distort more than a low compliance membrane.
Modern mobile phones should be able to produce two types of acoustic signals, low level phone type signals in phone operation mode, typically the voice of the person to whom the user of said portable communications device is communicating, and high level alert type signals in alert operation mode. In order to produce acoustic output signals at these two different levels two different transducers have traditionally been used. However, since the reproduction demands of the alert signal have risen in connection with polyphonic ring signals, and since the importance of small sizes of the mobile phones has increased, mobile phones containing one speaker only have been presented. In order to produce the two levels of acoustic signals, the same transducer is excited with two different electric signal levels. Also, in the phone mode the mobile phone typically held the against user's ear, whereas in the alert mode the mobile phone is typically placed so that the speaker can send the acoustic signals into free air space. This difference of what the speaker faces in the phone mode as compared to the alert mode, means that the mobile phone experiences two different acoustic impedance scenarios.
International application published as WO01/33904 presents an attempt to minimize such impedance differences, by introducing various openings in a housing containing the speaker, in order to achieve a system that is so called “leak-tolerant”, i.e. tolerant to air leakages between a user's ear and the face of the phone designed to be held against said user's ear. However, this kind of approach gives rise to distortion either when the transducer is driven in alert mode or when it is used in phone mode. This is mainly due to impedance mismatch, and the approach to minimize the influence of the user's on the performance of a phone is not adequate in order to achieve an essentially distortion-free performance in both alert and phone mode. Furthermore, the leak tolerant solution works comparatively well with a low acoustic compliance speaker, but as the dimensions of the speaker decreases the acoustic compliance increases. The introduction of a hole between the front and back of speaker also increases the harmonic distortion of the system due to a higher acoustic load of the membrane.